When drilling for oil and gas it is desirable to be able to steer the drill bit, i.e. to move the drill bit along a chosen path, so that the drill bit does not have to follow a path determined only by gravity and/or the drilling conditions.
One method for steering a drill bit is to utilise a steering component such as that described in our published European patent 1 024 245. That steering component allows the drill bit to be moved in any chosen direction, i.e. the direction (and degree) of curvature of the borehole can be determined during the drilling operation, and as a result of the measured drilling conditions at a particular borehole depth.
Another method of steering a drill bit is to use a deflection member. The deflection member is located close to the drill bit and has a fixed or adjustable deflection which will tend to steer the drill bit in a direction dependent upon the orientation of the deflection. The deflection member may for example be a bent housing, or it may cause the drive shaft or drill bit to deviate from the centre of the borehole being drilled. When it is desired to drill a linear (or more linear) section of borehole the deflection member is rotated so as to continuously change the orientation of the deflection and therefore to cancel out the tendency for the borehole to curve in one direction. Rotation of the deflection member may be effected by way of a downhole motor or by way of the drill string.
UK patent applications 2 435 060 and 2 440 024 both describe methods of steering a drill bit by way of a controllable deflection member, the deflection member comprising a bent housing. The bend is provided in the housing of a downhole motor which lies immediately behind the drill bit. The drill string is rotated and there is a rotatable connection between the drill string and the housing of the downhole motor. A clutch mechanism is provided within the rotatable connection, the clutch mechanism controlling the orientation of the housing and consequently the orientation of the bend.